1. Field of the Invention
The present invention relates to a two-stroke cycle combustion engine of an air scavenging type which may be used as a drive source for a portable work machine such as, for example, a brush cutter.
2. Description of the Prior Art
The two-stroke cycle combustion engine of a scavenging type has been well known, in which air used for leading scavenging is supplied into the combustion chamber after it has been temporarily introduced into a leading portion of the scavenging passage during the scavenging stroke. In this combustion engine, in order to secure a stabilized rotation during the idling condition, an air valve in the air passage is closed and only the air/fuel mixture is introduced into the crank chamber from an air/fuel mixture passage so that the air/fuel mixture of an optimum concentration can be supplied from the scavenging passage into the combustion chamber during the idling condition. On the other hand, if a rapid acceleration is performed from the idling condition to a full throttle condition, an air valve is brought to a fully opened condition at once.
In this respect, because of a low speed rotation taking place with a mixture valve in the air/fuel mixture passage fully opened, the negative pressure at the venturi tube within the carburetor is low and, therefore, the amount of the air/fuel mixture supplied from the carburetor towards the crank chamber is still insufficient. Accordingly, the air/fuel mixture pooled within the crank chamber is substantially supplied to the combustion chamber. But since a substantial amount of a scavenging air is introduced into the combustion chamber immediately after the full opening of the throttle, the air/fuel mixture tends to be leaned within the combustion chamber. For this reason, at the time the engine operating condition begins to change from the idling condition to the rapid accelerating condition, the air/fuel mixture of a concentration required for the rapid acceleration will not be supplied to the combustion chamber and, therefore, an acceleration failure and/or a failure of the combustion engine to rotate is apt to occur.
In view of the above, it may be contemplated to use, during the idling condition, the air/fuel mixture which has been leaned beforehand, but this appears to result in an increase of the idling opening, accompanied by opening of the air valve by which air enters into the combustion chamber in a quantity enough to render the rotation to be instable. Also, where a start operating mechanism of a lift-up type is employed in which a needle valve is lifted to increase a fuel supply, the amount of lift decreases in a quantity corresponding to the increase of the idling opening and, therefore, the air/fuel mixture during the start of operation of the combustion engine will not be enriched sufficiently, resulting in reduction in engine startability.
In contrast thereto, the two-stroke cycle combustion engine has been suggested of a design, in which an auxiliary passage for supplying a leading air to the scavenging passage during full opening or minimum opening of the air valve is employed in the air passage so that the amount of air in the air/fuel mixture can be reduced in a quantity corresponding to the amount of air flowing through the auxiliary passage during the idling condition to thereby allow the air/fuel mixture of an enriched concentration to be supplied into the crank chamber (See, for example, the Patent Document 1 listed below). In this two-stroke cycle combustion engine, since the amount of the air flow in the air/fuel mixture passage is reduced in a quantity corresponding to the amount of air flowing through the auxiliary passage during the idling condition, the concentration of the air/fuel mixture flowing within the air/fuel mixture passage tends to increase.
Accordingly, despite that during the idling condition, the respective amounts of air and fuel to be introduced into the combustion engine are set to values substantially equal to those hitherto employed, the air/fuel mixture, which is more enriched than that hitherto employed, is supplied to the combustion chamber at the time the engine operating condition begins to change from the idling condition to the rapid accelerating condition. In view of this, even if this air/fuel mixture is leaned in admixture of a portion of the leading air, the concentration of the air/fuel mixture supplied into the combustion chamber is maintained at a value required for the rapid acceleration and, therefore, the combustion engine can be smoothly accelerated.
[Patent Document 1] JP Laid-open Patent Publication No. 2007-239463
The two-stroke cycle combustion engine disclosed in the Patent Document 1 referred to above has, however, been found having such a problem that since the flow within the auxiliary passage and the mixture passage, where the mixture valve is disposed, relies on the negative pressure developed inside the crank chamber and, during the idling condition, the air flows into the air passage through the auxiliary passage and the amount of air flowing through the mixture throttle valve tends to change under the influence of air flowing through the auxiliary passage. Accordingly, the amount of fuel to be supplied, which is determined depending on the amount of air flowing through the mixture valve, tends to fluctuate, resulting in reduction in rotational stability of the combustion engine during the idling condition.